"Today the government has issued a critical rule that will permit the Department of Homeland Security to appropriately hold families together and improve the integrity of the immigration system," said Mr McAleenan.

"This rule allows the federal government to enforce immigration laws as passed by Congress and ensures that all children in US government custody are treated with dignity, respect, and special concern for their particular vulnerability."

The American Civil Liberties Union (ACLU) condemned the policy, saying: "The government should not be jailing kids, and certainly shouldn't be seeking to put more kids in jail for longer."

What does the new regulation do?

It aims to replace a decades-old court agreement - known as the Flores settlement - that both limited how long the government could hold migrant children in custody and specified the level of care they must receive.

A 2015 legal ruling on this issue specified that children should be held for no more than 20 days.

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Media captionLast month a Guatemalan mother spoke about the death of her 21-month-old baby daughter in a US detention centre

Under the new rules, the government could send families caught crossing the border illegally to family residential centres for the duration of their immigration cases.

President Trump has repeatedly complained about the "catch and release" of migrant families under the old rules.

Last summer, the Trump administration separated children from their parents as a means of circumventing the Flores settlement.

The children were held by the Department of Health and Human Services while the adults were imprisoned while awaiting trial for breaking immigration laws.

In June 2019, a Trump administration lawyer was admonished by federal judges in San Francisco after she argued that the Flores settlement did not require the government to provide detained children with soap or toothbrushes.